Since the discovery of a tumor-associated antigen, various methods for measuring the level of carcinoembryonic antigen (CEA) in sera or plasma have been described. Several of these methods employ radioimmunoassay (RIA) procedures. Generally, two different assay approaches have been suggested. In one approach, the specimen, e.g., plasma, is pretreated to separate the carcinoembryonic antigen material from other materials present in the specimen. In such an approach, a glycoprotein solvent, such as perchloric acid, is employed followed by time consuming dialysis to separate the soluble CEA-containing material from the other material present in the specimen. In the second general approach, no pretreatment of the specimen with such a solvent is employed. Instead, the specimen (e.g., plasma) is subjected to a so-called double-antibody treatment. This latter method has the disadvantage of requiring highly specific antibodies that are not readily available and also a relatively long period of time (two or three days) to perform the assay.